There is always intense discussion about the use of spray adjuvants among farmers and agronomists. As an independent agronomist I feel that they are often used unnecessarily, however in many situations they can improve the effectiveness of sprays and even allow dose rate to be reduced by improving the effectiveness of the chemical.
Not all adjuvants do the same job however, and many perform several roles, such as Nu-Film P and Agral. For this reason it is important to understand the different types and when to use them.
Most ‘Adjuvants’ are surfactants, or non-ionic wetters as they are also known. They work by reducing the surface tension of water, allowing smaller droplets that lay flatter on the leaf. In short, they improve spray coverage and droplet characteristics. Most have an oil base designed to perform two functions; to have an affinity to water (the spray droplet), and an affinity to fats (the waxy leaf).
Organosilicones – very effective spreaders and penetrants, although do show toxicity to some insect species. Avoid using when Bees are likely to be in the crop. Can degrade rapidly in acidic or alkaline spray solutions. Very good at improving rain fastness, but there is evidence that they are so effective that they can open the leaf to fungal infections.
Non-ionic surfactants are not always desirable. They can cause problems with membrane permeability when used at too high rates, so caution is needed to avoid leaf damage.
Mineral and Vegetable Oils work by either disrupting the waxy cuticle on the leaf surface, or reducing the speed of evaporation. They also provide a medium to extend the life of pesticides on the leaf surface. Herbicides do vary in their response to oil adjuvants; glyphosate is highly water soluble, so using oil adjuvants can often REDUCE the effectiveness of glyphosate.
Bentazone on the other hand is a non systemic, contact herbicide that responds well to oils, allowing lower rates of product to be used. Some Group A herbicides such as pinoxaden have to be mixed with vegetable based oils, and come supplied with a specific adjuvant in certain markets. Many actually contain adjuvants in the can ready formulated so are not always necessary.
Oils are the most likely to cause leaf scorch, especially in hot weather, and tend to be most aggressive on the leaf wax. If you are in a situation where you are forced to spray in less than ideal conditions, in the heat of the day, reducing the rate of an oil adjuvant is often a sensible step.
Vegetable based oils are most effective on dose rate sensitive sprays – clodinafop and atrazine for example – where a lower rate of chemical can be used on while still delivering the maximum amount of chemical inside the leaf.
pH buffers and water conditioners
These are used to control the pH of the spray solution. Ammonium Sulphate is commonly added to glyphosate to prevent the salt reacting with ions in the water such as calcium. These MUST be added to the tank first and circulated, to lockup the cations before the glyphosate is added.
Many chemicals can suffer a similar fate, but are ready co-formulated with ammonium sulphate (glyphosate is antagonistic to the aluminium used in the manufacturing process of ammonium sulphate, hence the need to add in separately in many case).
In other cases such as 2,4-D the chemical uptake into the plant is simply increased by reducing the pH towards 5 or lower. Propionic acid is commonly used to acidify spray solutions, although it is important to check – many sulfonylureas form salts at low pH which can block the sprayer or reduce herbicide performance. Do NOT acidify every spray mix without checking!
Drift control agents and soil binding adjuvants are becoming more common, mainly for use with soil acting pre emergence herbicides. Think about apply a soil herbicide ahead of maize emergence – the wind speed on the soil surface is high as there is no plant cover, and the chemical needs to bind to the soil before it rains.
Pod Sealants are not technically an adjuvant as they are often applied alone, to seal the pods closed on peas, canola or any other podded small grain. Based on latex polymers they literally coat and seal the pods closed, but not too tight that the harvester can’t break the tension. In the early days these had a reputation for clogging up combines and sticking around the rear beater, but new formulations have shown consistent yield increases from reduced harvest losses in independent trials.
How adjuvants can dramatically influence the droplet size and spray deposition!!! A very useful image from the GRDC Adjuvant handbook – see the link below.
- A chart showing the different groups of adjuvants: http://www.desangosse.co.uk/products/spray-enhancers/adjuvants
- The GRDC’s very thorough Handbook Guide to adjuvants: https://grdc.com.au/resources-and-publications/all-publications/publications/2015/04/adjuvants-booklet